1. Field of the Invention
The present invention relates to a method of producing a liquid developer and a liquid developer produced by the method.
2. Description of the Prior Art
As a developer used for developing an electrostatic latent image formed on a latent image carrier, there are known two types. One type of such a developer is known as a dry toner which is formed of a material containing a coloring agent such as a pigment or the like and a binder resin, and such a dry toner is used in a dry condition thereof. The other type of such a developer is known as a liquid developer which is obtained by dispersing toner particles into a carrier liquid having electric insulation properties.
In the developing method using such a dry toner, since a solid state toner is used, there is an advantage in handleability thereof. On the other hand, however, this method involves problems in that contamination is likely to be caused by dispersal of toner powder and toner particles are likely to be massed together in a cartridge. Further, in such a dry toner, since aggregation of toner particles is likely to occur in the producing process thereof, it is difficult to obtain toner particles each having a sufficiently small diameter. This means that it is difficult to form a toner image having high resolution. Furthermore, there is also a problem in that when the size of the toner particle is made to be relatively small, the problems resulted from the powder form of the dry toner described above becomes more serious.
On the other hand, in the developing method using the liquid developer, since aggregation of toner particles in the liquid developer is effectively prevented, it is possible to use very fine toner particles and it is also possible to use a binder resin having a low softening point (a low softening temperature). As a result, the method using the liquid developer has the features such as good reproductivity of an image composed of thin lines, good tone reproductivity as well as good reproductivity of colors. Further, the method using the liquid developer is also superior as a method for forming an image at high speed.
Conventionally, such a liquid developer is produced by a grinding method in which toner particles are produced by grinding a resin (see JP-A No. 07-234551, for example) or a polymerization method in which monomer components are polymerized in a solution having electric insulation to produce resin particles which are not soluble in the electric insulation solution (see JP-B No. 08-7470, for example).
However, these conventional liquid developer producing methods accompany such problems as described below.
In the grinding method, it is difficult to grind toner particles to a sufficiently small size (e.g. 5 μm or less) by which the above described effects resulted from the use of the liquid developer can be exhibited sufficiently. Namely, it takes very long time or it requires very large energy in order to obtain toner particles having such a sufficiently small size, thus leading to extremely low productivity of a liquid developer. Further, in the grinding method, a particle size distribution of toner particles is likely to be large (that is, there are large variations in particle size), and the shapes of the toner particles are liable to be irregular and nonuniform. As a result, obtained toner particles are likely to have variations in their properties among the toner particles.
Further, in the polymerization method, it is difficult to set polymerization conditions properly. This means that it is difficult to form toner particles having a desired size. Further, it is also difficult to make the variations in the size of the toner particles sufficiently small. As a result, stability of quality of a liquid developer and reliability thereof are likely to be low. Further, since the polymerization method requires a relatively long time for the formation of the toner particles, the productivity of the liquid developer is not so high.
Furthermore, since the carrier liquid having electric insulation properties (namely, the insulation liquid) used in the above described method has low compatibility to components of the toner particles, aggregation between the toner particles is likely to occur, whereby making it difficult to keep a good dispersion state of the toner particles in the carrier liquid for a long period of time. Therefore, it is difficult to preserve the liquid developer for a long period of time.